The present invention relates to a plant for treating drill cuttings obtained during oil and gas drilling, in particular but not only in connection with subsea oil- and gas wells. The plant performs a safe and permanent depositing of drill cuttings without contamination of the environment.
Many different approaches have been made to solve the problem related to disposal of drill cuttings which are brought to the surface by a drilling rig during a drilling operation. Great efforts have been made to clean the drill cuttings to regain valuable components therefrom. The purified drill cuttings have then been dropped back into the ocean. Some earlier suggested solutions may be mentioned such as: cleaning by washing with sea water; formation of pellets; and depositing in a wide, vertically arranged tube which is filled with drill cuttings from above while optionally collecting oil components liberated in the tube. An attempt has also been made to grind drill cuttings in a mill with frictional heat developed during the grinding process used for removing oil components from the drill cuttings by evaporation.
The present invention instead is aiming at obtaining a complete and final treatment of drill cuttings which have surfaced during the drill operation. The drill cuttings are not purified for deposition; on the contrary, the drill cuttings are disintegrated by crushing, i.e., all the constituents in the drill cuttings are kept intact while the crushing is repeated until particles above a certain predetermined size no longer exist. Then these particles are used to obtain a dispersion and/or emulution, (hereinafter referred to only as a dispersion), in water. This dispersion may finally be forced back into subsea structures by injection, preferably into the same structures from which the drill cuttings initially came with pressures as required according to the structure of the subsea formations at the depths where the injection is undertaken.
Earlier known methods for depositing drill cuttings have first of all led to great contamination problems. These problems are increasing and have recently overwhelmed the cost and time aspects of these proceedings. Earlier on it also has been difficult to find large and suitable deponating sites giving a safe and permanent deposition of the materials in question. By using the same formations in which the materials initially have been kept for thousands or millions of years as a re-storing place, a safe storing place is garanteed. The volume of such a space also are more than satisfactory as large amounts of useable materials have been removed before the re-injection takes place.
As far as we know, it has earlier only been attempted to deposite waste liquids by injection. When solid matters have been included it has always resulted in clogging problems. As far as we know attempts including re-injection of solid materials back to porous formations never have been tested in practice. The solid materials are anticipated to clog the structure and will therefore prevent further infusion. Since such tests are very expensive, they will not be carried out unless a good result is anticipated. The object of the present invention is to obtain a plant for treating and injecting of drill cuttings and other solid/liquid combinations wherein earlier known problems are solved and where expenses and treatment time are lower level than earlier methods obtained by injecting liquids.
The object of the present invention is to obtain a plant for treating drill cuttings and other waste materials in an inexpensive, efficient and fast manner while the above mentioned disadvantages are avoided. This is obtained by designing the plant according to the claims below.
The plant according to the present invention leads to additional advantages as explained below and is designed for treating drill cuttings and similar waste materials, in particular materials surfacing during the drilling for gas and oil.
The plant according to the present invention is characterized in that it comprises:
at least one crushing machine being provided with at least two input ports; the first input port being especially arranged for receiving a compound having a high percentage of solids, which input port leads directly towards the rotor blades of the crushing machine, while the other input port is arranged to receive a compound having a high percentage of liquids and to lead this compound in between the rotor blades and the stator rim of the crushing machine; and being provided with an output port, PA1 a mixing receptacle (4) having the output port from the crushing machine arranged as its input port, PA1 at least one additional container of which the first one is arranged adjacent to the mixing receptacle and is separated from same by a partition, PA1 at least one feed-back arrangement adopted for guiding at least the solids from the mixing receptacle back to one of the input ports of the crushing machine, and PA1 at least one injecting device arranged to inject a liquid having solid particles dispersed therein from the last one of the additional containers into a formation in the ground.
According to a specific and simple embodiment of the invention the plant comprises one crushing machine and two containers. The crushing machine is provided with two input ports, one port for inputting solids and one port for inputting liquids, and having one output port leading from the crushing machine to a first mixing receptacle. To this mixing receptacle there is also applied one input port for water, and this is, in parallel with the output port from the crushing machine, led down into the first mixing receptacle. From this mixing receptacle, and preferably from the lower part of same, a feedback pipe transports one portion of the content and in particular a portion having relatively large particle size, back to the input port of the crushing machine. This feedback should preferably be arranged in such a manner that it also results in an agitating of the contents in the first mixing receptacle and may in addition be designed so that the thinner portions of the feedback materials do not necessarily have to pass through the complete process in the crushing machine.
An additional crushing machine may also be provided for the feedback material to treat this in a somewhat different manner. The feedback material may for instance be crushed down to a more fine-meshed particles than in the first mentioned crushing machine.
When the process is started the mixing receptacle will soon be filled up with a mixed compound comprising crushed drill cuttings and water, and this compound will be in continuous motion. The mixing receptacle is provided with an open top and shall therefore flow over when filled up. It should also be mentioned that the mixing receptacle preferably is provided with a slanting bottom and that the feedback compound is collected from the lower part of this slanting bottom which ensures that all the largest particles will be fed backwards in the process and shall not end at this stage. The feedback process preferably is undertaken by means of a vane pump of the centrifugal type having an extraordinary large input port, for example a so called agricultural pump.
The material flowing over from the mixing receptacle will flow further into a second container which may be designated as a "sander" or a sand removing tank. From the "sander" the material is pumped through a sand removing unit which may comprise one or more hydrocyclones, preferably arranged in parallel. From the bottom of these hydrocyclones there will fall out humid particles having a non-classified size, while at the top of the hydrocyclones will flow water including very small-sized particles from the crushed drill cuttings dispersed therein. The dispersion which comprises fine drill cuttings together with water, may then be conveyed to a third container which may be referred to as an output tank. From the output tank the dispersion is pumped further to the last tank which may be referred to as an injection tank. The pump used may also here preferably be a so called agricultural pump. If the output tank is flowing over, the overflow shall be guided through an overflow connection back to the sand removing unit.
Considering particles which fall out below the hydrocyclones, these will fall down into a collecting pan after passing through the meshes in an optional, vibrating screen which passes only particles having a size less than a predetermined value given by the mesh size. The particles which fall through the screen are also led down in the output tank, i.e. together with the dispersion entering from the upper part of the hydrocyclone(s). The larger particles which do not pass through the meshes of the vibrating screen, are transferred to a conveyer for solid particles and via this conveyer the larger particles are led to further stages in the process. However different alternatives exist for further handling of those larger particles, within the scope of the present invention.
The plant may for instance be equipped in such a manner that the conveyer transports the larger, solid particles directely back to the input port of the crushing machine so that the particles have to pass through the process an additional time.
A further possibility is letting the conveyer guide the particles to a different kind of crushing machine than the one already used. This second crusher may for instance be a mill particulary designed for sand milling, and from this mill the sand obtained may again be guided down into the mixing receptacle together with the earlier mentioned crushed drill cuttings which have passed the initial crushing machine.
Still a further possibility shall be mentioned as the larger, solid particles may be guided to a conveyer of the sand-washing type, where the particles are cleaned by water flowing in the opposite direction, so that the particles are completely pure when arriving at the opposite end of the conveyer. Using this technique the separated larger and solid particles may, after cleaning and washing, be led directly back into the water surrounding the platform, without any danger for the environment, while the separated waste liquid is guided backwards to the output tank.
As mentioned above the screen may represent an optional solution. If a screen is not used, all the underflow from the hydrocyclone(s) may be guided back to the mixing receptacle or to the sand removing tank, preferably close to a pump input carrying the underflow again into a feedback process or a circulating process according to the claims below.
If the dispersion which was pumped from the output tank to the injection tank now is considered, this dispersion shall preferably be collected in batches. Because of this it is preferred to use at least two injection tanks with only one injection tank is filled at the time. The dispersion which arrives at the injection tank has preferably none, or at least only a few particles having a diameter above 75 micron. If the dispersion comprises redundant water this may be guided via an overflow back to the output tank or even back to the sand removing tank or the mixing receptacle.
When an injection tank is filled up by a dispersion having the desired qualities, the filling of this tank is stopped while filling of the next tank begins. While the next tank is being filled, the content in the first injection tank which now is filled up, may be guided into a piston pump generating the pressure required to pump this fine-grained dispersion into the subsea structures according to existing depths and existing porousity of the formations at this site. Thereafter a pump, which may be a high pressure pump, and a tube/pipe system enduring the pressure in question are used to empty the contents from the injection tank and squeeze it into a suitable subsea structure. Then this injection tank will again be ready for a new filling operation. The pump may possibly be a pressure booster.
The plant described, in accordance to the present invention, leads to a high speed treatment of drill cuttings. This treatment may be continous even if the dispersion is handled in batches since feedback of redundant material from earlier stages in the process continually prevent clogging in the plant. Bottlenecks in which the manufacturing of the dispersion comes to a stop, do not exist. To this end it is important that all the pumps are open vane pumps having large input and output ports which are not easily clogged by course materials passing therethrough. To obtain such a function the crushing machine may also be designed in such a manner that objects having a size exceeding the pump capacity are not allowed to pass.
It should also be mentioned that the feedback of compound from the mixing receptacle preferably is undertaken in such a manner that one portion of the compound is guided back to a gun or jet, which at high pressure but with a certain dispersive effect from a movable nozzle, may inject a stream of feedback material into the mixing receptacle generating a whirling and stirring process in the mixing reseptacle. A different portion of the feedback compound may be led to the crushing machine between the rotor and the stationary outer rim, which gives a so called autogenous crushing since different sized particles are crushed when hitting each other at high velocities and high kinetic energy. The larger feedback particles are guided directly into the main input port of the crushing machine.
In this connection it should be mentioned that a crushing machine preferably could be operated more efficiently when additional grinding elements are put into the system. Such grinding elements may consist of small steel balls or elements having a more random shape such as short pieces of reinforcement rods. Such grinding elements circulate in a closed loop within the plant and do not pass through to the output port. Such grinding elements therefore shall only be consumed by regular wear and abrasion.
The working principles of this plant will be understood from the description above. The principal stages in the process are described below.
Drill cuttings which have not been treated, possibly mixed up with sand, water and slurry or mud from the drilling operation is guided through the crushing machine. Additional water may be added to obtain a compound of suitable consistency. In this compound there may partly be undertaken one or more feedback operations in directions towards the crushing machine and/or partly one or more circulations in the forward direction towards the injection tanks. Thus the mixed up compound will circulate within the plant as the larger particles are continuously forcedly crushed again and again, while the compound is continually thickening until a stable dispersion of desired consistensy, suitable for injection, has been obtained.